Interpretive Summary: Proper scheduling of irrigation water application is critical to efficient water use and extending limited water supplies. Crop water requirements depend on the size of the plant, or canopy cover, and the weather, or potential evapotranspiration. We developed relationships between crop water use and canopy cover for 3 horticultural crops. We also developed relationships between canopy cover and vegetation indices (NDVI) calculated from satellite images. These relationships allow us to estimate canopy cover from satellite data, and then to estimate crop water requirements over large areas with weather station data. These relationships will allow efficient and extensive prediction of crop water requirements and more efficient irrigation water use.
Key Words: Remote Sensing, NDVI, ET, Evapotranspiration, Crop water use, irrigation scheduling

Technical Abstract:
Crop water use can be estimated from reference evapotranspiration, ETo, calculated from weather station data, and estimated crop coefficients, Kc. However, because Kc varies with crop growth rate, planting density, and management practices, generic Kc curves often don’t match actual crop water use. Recent studies have shown that basal crop coefficients, Kcb, are related to crop light interception or canopy cover; and that canopy cover can be estimated for a wide variety of crops from remotely sensed observations of the normalized difference vegetation index, NDVI. Combined, these relationships could provide good estimates of Kcb from satellite or aerial data for a wide variety of crops over large areas. When combined with ground based ETo measurements and general knowledge of irrigation methods, crop water use can be estimated for individual fields and for large regions. Because NDVI data are generally available only intermittently during the season, simple plant growth models can be used to interpolate canopy cover between vegetation index measurements, and to extrapolate crop canopy cover into the future. This system may improve estimates of crop water use compared to traditional FAO-56 methods and provides an alternative to remotely-sensed estimates of ET that use thermal data with surface energy balance calculations.